Aromatization of naphtha



`lune 4, 1957 R. G. HAY ET AL AROMATIZATION 0F NAPHTHA Filed OIM.. 19.1954' www United States Patent AROMATIZATION F NAPHTHA Russell G. Hay,Fox Chapel, and Ralph W. Helwig, Oakmont, Pa., assignors to GulfResearch c Development Company, Pittsburgh, Pa., a corporation ofDelaware Application October 19, 1954, Serial No. 463,239

1 Claim. (Cl. 2611-668) This invention relates to the preparation ofaromatic hydrocarbons and more particularly to a process for thedehydrogenation of petroleum hydrocarbons to form aromatic hydrocarbonsand the recovery of the aromaticA compounds from reaction productshaving lower boiling points.

Aromatic hydrocarbons in the gasoline boiling range are valuable as fuelfor internal combustion engines because of their high anti-knock rating,and also are valuable solvents. Specic aromatic hydrocarbons such asbenzene, toluene, xylene and ethylbenzene in substantially puresolutions are important raw materials in the synthesis of plastics andother organic compounds. The aromatization of less valuable hydrocarbonsof higher hydrogen to carbon ratio than aromatic hydrocarbons isaccomplished by passing the non-aromatic hydrocarbons in contact with asuitable catalyst at an elevated temperature at which dehydrogenationand ring closure, if necessary, occur. For example, it has beensuggested in the prior art that the non-aromatic hydrocarbons can bepassed in contact with a composite of alumina and chromia attemperatures in as wide a range as 750 to 1200 F. to form aromatichydrocarbons.

The aromatization of non-aromatic hydrocarbons is a highly endothermicreaction; hence, it is necessary to supply large quantities of heat tothe reaction to maintain the optimum reaction temperature. Large volumesof hydrogen are formed in the aromatization reaction, which hydrogen, inaddition to the other reaction products, causes high velocities ofreaction products and entrainment `of substantial amounts of catalystlines from uidized catalytic aromatization processes.

This invention resides in a uidized catalytic process, and apparatustherefor, for the preparation of aromatic hydrocarbons from non-aromatichydrocarbons of higher hydrogen to carbon ratio, and the separation andrecovery of the hydrocarbons boiling in the range of the aromatichydrocarbons from the entrained catalyst fines and reaction products oflower boiling point. Throughout this specification, the term aromatichydrocarbons means benzene and alkylated benzenes such as toluene,Xylene and ethylbenzene. Aromatic hydrocarbons having more than onebenzene ring are not includedwithin the scope of the term aromatichydrocarbons as herein used.

The single figure of the drawings is a schematic ow sheet illustrating aprocess and apparatus suitable for use in this invention. y

Referring to the drawing, charge stock is delivered from a storage tankthrough a line 12, pump 14, heat exchanger 16 and furnace 18, to atransfer line 20 from which the charge stock is discharged into areactor 22 in which a tluidized bed of catalyst is maintained. Suitablecharge stocks are normal and branched paraihns and olens of at least sixcarbon atoms and naphthenic hydrocarbons preferably having six or morecarbon atoms in the naphthenic ring. The charge stock can besubstantially a single compound, or a mixture of hydrocarbons having thesame number of carbon atoms per molecule for the preparation of aspecific aromatic compound, as in the aromatization of normal hexane andhexene to produce benzene. Also suitable for charge stocks are mixturesof hydrocarbons of hydrogen to carbon ratios higher than those ofaromatic compounds having different numbers of carbon atoms permolecule, and particularly mixtures of petroleum hydrocarbons boiling inthe gasoline range to produce .a mixtureof different aromatichydrocarbons. The term naphtha is employed in this specification todesignate'non-aromatic hydrocarbons of the proper boiling point suitableas charge stocks in the process of this invention.

Recycle gas consisting principally of hydrogen from a line 24 in amountsup to about 6000 standard cubic feet per barrel of naphtha is mixed withthe charge stock prior to entrance into the furnace 18, wherein themixture is heated to approximately the reaction temperature. The chargestock discharged from furnace 18 picks up regenerated catalyticdischarged from the bottom of a standpipe 26 and delivers it to thefluid bed within vthe reactor 22.

The catalyst within the reactor 22 can be any conventional catalyst forconversion of naphtha to aromatic hydrocarbons in finely divided form inwhich the major weight proportion of the particles falls within therange of about 20 to 100 microns. Examples of suitable catalysts arecomposites of alumina and chromia and alumina and ,molybdena Thealumina-chromia catalysts, which are preferred for the preparation ofbenzene, can be prepared by the impregnation with chromium nitrate of anactivated alumina, or an activated alumina impregnated with smallamounts of silica, followed by thermal decomposition of the chromiumnitrate to form an aluminachromia composite. Other suitablealumina-chromia catalysts are prepared by coprecipitation of alumina andchromia followed by washing, drying and calcining. Preferred catalystsare alumina-chromia catalysts contain ing about 5 to 30 percent chromiawhich are further improved by treatment with sodium hydroxide orpotassium hydroxide.

The finely divided catalyst particles are suspended in a uidiz'ed densephase by the ascending vapors of charge stock, hydrogen-containing gasesadded to the charge stock, and reaction products. The temperature in thereactor may range from as low as 750 F. to as high as l200 F.; however,itis preferable that the temperature within the reactor be within therange of 900 to 1150 F., and still more desirably between 950 and 1100"F. The pressure is maintained as low as possible consistent with themaintenance of satisfactory throughputs through the reactor to improvethe yield of aromatic hydrocarbons. Pressures below about 25 p. s. i.g., and preferably about 5 p. s. i. g., are employed. The space velocityof the charge stock is between 0.1 volume of liquid feed per volume ofcatalyst per hour and 10 volumes per volume per hour.

Deactivated catalyst is withdrawn from the reactor 22 through astandpipe 28 and discharged into a transfer line 30 in which :thecatalyst is transported by a stream of air introduced through Va supplylline 31 from a source, not

shown, to a regenerator 32. A fluidized bed of catalyst i such as fueloil, into the regenerator through a =line 36 to heat the regeneratedcatalyst to a temperature sufeiently high .to convey substantial heat tothe .reactor in the form of sensible heat of the catalyst. Thetemperature in the regenerator is maintained at 950 to l250 -Flue gasesfrom the regenerator pass through a cyclone separator 3 8, in whichentrained catalyst .particles are separated and returned to. the liuidbed, and are dis- Charged through line 40 to a stack.

.Gaseous reaction products from the uid bed within reactor 22 passthrough a separator 42, in which the larger of the entrained catalystparticles are separated from `the reaction products, and then aredelivered through line 44 to a scrubbing tower 46. Prior to. theirintroduction into. the scrubbing tower 46, the reaction products arequenched ,with a relatively cold stream of quench oil from line 48. Themixture of quench oil and reaction produqts enters the bottom ofscrubbing tower 46 through a line 5 0. The reaction products, whichremain ill the gaseous phase, ow upwardly through the scrubhing tower incountercurrent contact with descending scrubbing oil which cools thereaction products to a temperature suitable for further processing andremoves entrained catalyst fines from the reaction products. Thescrubbing tower is provided with suitable apparatus such as thebubblecap trays and disk and doughnut trays indicated, diagrammaticallyfor improving the contact between, quench oil and ascending gaseousreaction products.

The quench oil and catalystnes are discharged from the bottom ofscrubbing tower 46 through a line 52 and are circulated by means of apumpv 54 through a cooler 5.6'. A portion of the cooled quench oildischarged from Cooler 56returns via line 48 for quenching reactionproducts. A second portion of the. cooled quench oil passes :throughYlines 58 and 60 to. a point at approximately the middle of: thescrubbing tower 46. to provide a downward flow ofi liquidin the tower toscrub the reaction products. A third: portion of the quench oil passesthrough line 62 to a. continuous lter 64, or other suitable apparatus,in which the entrained catalyst particles are separated from the. quenchoil. The filtered quench oil is returned through lines 66. and cooler68- to the top of the scrubbing tower 46. The quench oil introduced intothe top oi; the.. scrubbing tower 46 provides catalyst-free reflux inthe top of the scrubbing tower which further cools the gaseous reactionproducts and cleans traces of catalyst which-.may be present from thegases.

T-he gaseous reaction products from scrubbing tower 46;ow through line70.to a compressor 72- in which the pressure on the gases is raised to apressure which will facilitate the absorption of the aromatichydrocarbons in an4 absorber oil. A pressure in the range of 35 to 75 p.s. i. is preferred. The compressed reaction products are then cooled inva. heat. exchanger 74 and introduced into the. bottom of an absorptiontower 76. A stream of* lean absorber oil is introduced into the Itop ofthe absorber. tower through linev 78A and descends through the towercountercurrently to the gaseous reaction products. Thedescendingabsorber oil extracts a higher boiling. fraction, hereindesignated as the aromatic fraction, which includes the aromatichydrocarbons and other hydrocarbons of approximately the same boilingpoint as the aromatic hydrocarbons, Ifrom the reaction products of lowerboiling-point, and is discharged as a rich oil from the. bottom ofabsorption tower 76 through a line 80. The reaction. products which arenot absorbed by the absorber oil, and which largely consistof hydrogen,are discharged from the. top of the absorption tower 76 throughline 82.into a gas line 84; The gas line 84 is connected withline 24 for therecycle of hydrogen to the reactor, Absorption tower 76 is providedwithbubblecap trays, perforatedftrays, ortotherapparatus-such as packing,toimprove the contact between the liquid andgaseous phases therein.

Rich. absorber` oil is directed through line 80 to a heat exchanger 83.-in which it is heated byheat exchange with lean absorber oil and a thendelivered through a line into the top of a stripper 86. Stripper -86 -isequipped with bubble cap trays, perforated trays, packing, or othermeans for providing etiicient contact between the descending rich oiland ascending gases which strip the aromatic fraction from the rich oil.Lean absorber oil is discharged from the bottom of the stripper 86through a line 88 to a pump 90. A portion of the lean oil is pumpedthrough a hcaterv92, which serves as a boiler, and then back to thestripper 86 through line 94. Heater 92 provides the. heat necessary forthe vaporization of the absorbed aromatic fraction lfrom the rich oil.Another portion of the lean oil passes from pump through a line 96, andheat exchanger 83, wherein it gives up a portion of its sensible heat tothe rich absorber oil, and then passes through a line 98 to heatexchanger 16 in which the Ilean oil is further cooled by heat exchangewith the charge stock. The cool lean oil from heat exchanger 16 isfurther cooled in a cooler 100 and delivered through line 78 to the topof the absorber- 7-6 for the absorption of the aromatic fraction fromthe lower boiling point reaction products. Makeup absorber oil is addedto the system as required through line 101.

Gaseous reaction products stripped from the absorption oil in stripper86 are discharged from the top of the stripper through aline 102 andpass through a cooler 104 to an accumulator 106. Gases separated fromthe reaction products in the accumulator 106 are delivered through line108: tothe gas line 84. Liquid reaction products are removed' from theaccumulator 106 through a line 1'10and delivered to apparatus, notshown, for separation ofthe. aromatic hydrocarbons from non-aromatichydrocarbons ot? substantially the same boiling range. The nonaromatic-.hydrocarbons separated from the aromatic hydrocarbons may be recycledthrough the reactor 22, if desired.

The following specic embodiment of this invention is described' indetailto illustrate the process of this inventi'on for the preparationof benzene 4from a specic charge stock.

A charge stock consisting of approximately 80 percent by volumeof`normal hexane, 13% percent by volume methylcyclopentane, 5 percent byvolume isohexanes, and ll/ percent by volume benzene is heated to atemperature of. 1022" F. and is introduced into a reactor which ismaintained at the same temperature. The charge stock is passed throughthe reactor at a liquid space velocity off 1.0 volume per volume ofcatalyst per hour. The pressure in the reactor is regulated at 5 p. s.i. g. Hydrogen is recycled to the reactor by introduction into thecharge stock at the rate of 1500 standard cubic feet per barrel ofcharge prior to final heating of the charge to the reaction temperature.A lluidized bed of coprecipitated: alumina-chromia catalyst containingapproximately 80.percent alumina and 20 percent chromia is maintainedinthe reactor. l

Gaseous reaction products together with entrained catalyst fines arewithdrawn from the reactor at approximately the reaction temperature andquenched with a quench oil' at a temperature of 150 F. to produce amixture at a temperature of approximately 500 F. The quench oil'- is agas-oil of approximately 200 molecular Weight. The quenched mixture isintroduced into a tower. in which it isscrubbed with additional quenchoil. Gaseous reaction products pass upwardly through the scrubbing-towerand are discharged therefrom at F. Quench oil= and entrained catalystlines are withdrawn fromA the-bottom of the tower and are cooled atapproximately F. A portion of the cooled quench oil withdrawnfromthebottom of the tower is recirculated continuouslythrough the scrubbingtower. action products are also washed with a catalyst-free quench oilfor further cooling and scrubbing of the reaction products.

The scrubbed gaseous reaction products are compressed The gaseous re-vto a pressure of approximately 40 p. s. i. g. and cooled to atemperature of 130 F. The aromatic fraction of the reaction products isabsorbed in a light gas-oil of approximately 200 molecular weight. Aportion of the unabsorbed gases, which consists principally of hydrogen,is recycled to the charge stock at the rate set forth above.

The rich absorption oil is heated to a temperature of approximately 250F. and introduced into a stripper tower operated at a pressure ofapproximately p. s. i. g. A reboiler at the bottom of the stripperprovides the heat required for stripping the aromatic fraction from theabsorber oil.

The quenching of the reaction products not only cools the reactionproducts to a temperature suitable for further processing, but providesan ecient separation of entrained catalyst fines from the very largevolume of gaseous reaction products. Recirculation of cooled quench oilcountercurrent to the ow of gaseous reaction products thoroughly scrubsthe gases prior to their discharge from the scrubbing tower. Erosion ofthe equipment in which the catalyst lines are separated from thereaction products, as well as cooling and other processing equipment,its minimized by the quenching and scrubbing procedure.

The details of the process have been set forth with respect to anembodiment of this invention for the purpose of illustrating a processin detail. It will be appreciated that speciiic operating conditionswill depend at least in part upon the details of the design of theequipment. Specific operating conditions will be dependent in someinstances upon other operating conditions. For example, the temperatureemployed in the absorber and stripper will depend upon the operatingpressure of those units and their fractionating efliciency. Thisinvention is not limited to such details except as set forth in theappended claim.

What we claim is:

A process for the preparation of aromatic hydrocarbons comprising mixinga recycle gas consisting principally of hydrogen with a naphthaconsisting essentially of non-aromatic hydrocarbons having at least 6carbon atoms per molecule in a ratio of about 1500 to 6000 standardcubic feet of recycle gas per barrel of naphtha to form a charge stock,passing the charge stock through a heated coil to heat the charge stockto a temperature of 900 to 1150 F., introducing hot regeneratedaromatization catalyst particles from a regenerator into the heatedcharge stock to form a suspension of the catalyst in the charge stock,delivering the suspension of the catalyst in the charge stock into thelower end of a reactor containing a fluidized bed of the aromatizationcatalyst maintained at a temperature of 900 to l F. and a pressure belowabout 25 p. s. i. g., withdrawing a stream of aromatization catalystfrom the uidized bed in the reactor and transferring it to aregenerator, burning carbonaceous deposits from the catalyst in theregenerator, the temperature in the regenerator being higher than thetemperature in the reactor and in the range of 950 to 1200 to supplyheat to the reactor upon transfer of the regenerated catalyst to thereactor, withdrawing reaction products from the upper surface of theuidized bed and passing them through a separator in which entrain/edcatalyst particles are separated from the reaction products and returnedto the iiuidized bed, passing the reaction products from the separatorthrough a line into a scrubbing tower adapted for countercurrent contactof gas and liquid and maintained at substantially atmospheric pressure,withdrawing a slurry of quenching oil and catalyst from the bottom ofthe scrubbing tower, cooling the withdrawn slurry, discharging a firstportion of the cooled slurry into the line for admixture with thereaction products between the separator and the scrubbing tower,returning a second portion of the cooled slurry into the scrubbing towerabove the point of admission of the reaction products, further coolingthe quenching oil in a third portion of the cooled slurry, separatingcatalyst from the third portion of the cooled slurry to form asubstantially catalyst-free stream of quenching oil, introducing thecatalyst-free quenching oil into the scrubbing tower at a point higherthan the point of introduction of the second portion of slurry toprovide clean reflux further cooling and cleaning the gaseous reactionproducts in the scrubbing tower, discharging gaseous reaction productsfrom the upper end of the scrubber and compressing them to a pressure inthe range of about 35 to 75 p. s. i. g., cooling the compressed gaseousreaction products and passing the min countercurrent contact with anabsorber oil to strip aromatic hydrocarbons from the recycle gas,returning recycle gas for admixture with the naphtha, and strippingaromatic hydrocarbons from the absorber oil.

References Cited in the le of this patent UNITED STATES PATENTS Cardwellet al Dec. 22,

